The present invention relates to a composite electrode containing an organic sulfide compound, which is employed for electrochemical devices such as batteries, electrochromic display devices, sensors, memories and the like, a method for producing the same, and a lithium secondary battery having a cathode comprising the composite electrode.
Since an electrically-conductive polyacetylene was discovered in 1971, an electrode comprising an electrically-conductive polymer has been intensively studied. This is because electrochemical devices such as lightweight batteries with high energy densities, electrochromic devices having large surface areas, biochemical sensors utilizing minute electrodes and the like can be expected to be realized by using the conductive polymer as the electrode material. The polyacetylene however has some disadvantages for practical use in the electrodes because of its chemical instability and, hence, research has been directed to other .pi. electron conjugated conductive polymers, which is relatively stable, such as polyaniline, polypirrole, polyacene, polythiophene and the like. Lithium secondary batteries using these polymers for their cathodes have already been developed. It is said that the energy density of each of these batteries is 40 to 80 Wh/kg. As an organic material which is expected to have a high energy density, there is, for example, an organic sulfide compound having a thiol group or thiolate group in the molecule.
As the organic compound, there is suggested, in U.S. Pat. No. 4,833,048, a compound represented by the formula M.sup.+ --.sup.- S--R--S.sup.- --M.sup.+ wherein R is an aliphatic or aromatic organic residue, S is a sulfur atom, M.sup.+ is a protone or metal cation. Plural of such compound can be bonded together through S--S bond by electrolytic oxidation and polymerized in the form of M.sup.+ --.sup.- S--R--S--S--R--S--S--R--S.sup.- --M.sup.+. The produced polymer may be returned to the original monomer by electrolytic reduction. A metal-sulfur type rechargeable battery constructed by combining a disulfide type compound with a metal(M), which supplies and captures a cation(M.sup.+), is proposed in the above-described U.S. Pat. specification. The battery has a energy density of 150 to 200 Wh/kg. Further, sulfur is proposed in U.S. Pat. No. 5,523,179 as the electrode material which gives a high capacity and large energy density of not less than 200 Wh/k.
However, the oxidation-reduction reaction of the sulfur proceeds slowly at room temperature and it is difficult to provide a higher current output. And, when constituting a battery with an anode comprising a metallic lithium, a lower voltage of about 2 V is only provided. Namely, there is a disadvantage that only a lower operating voltage of not higher than 2V may be provided, because the oxidation-reduction reaction proceeds slowly and the resistance to electrode reaction is high when such a battery is operated at room temperature. On the other hand, a lithium battery with a cathode comprising an oxide of a transition metal such as lithium cobaltate or the like provides a high operating voltage of higher than 3.6V. Such a battery, which has a high operating voltage, can operate an electronic device alone. However, when using a battery with a cathode comprising sulfur, it is necessary to connect two batteries in series to raise up the voltage.
On the other hand, among the organic sulfide compounds having a thiol or thiolate group in their molecules, the specified organic disulfide compound as described above gives a battery which provides a high voltage of not lower than 3V if combined with a metallic lithium anode. However, there is a disadvantage that an electrode capacity is decreased gradually as the oxidation-reduction (charge and discharge) of the battery is repeated. The reason for such a phenomenon is regarded as in the followings.
When the organic disulfide compound is oxidized (charged), an electrically-insulating polydisulfide compound which is poor in ionic conductivity is produced. The polydisulfide is poor in solubility to the electrolyte. On the other hand, an organic disulfide monomer, which is produced when the polydisulfide compound is reduced (discharged) to form a monomer, is highly soluble to an electrolyte. Accordingly, a part of the disulfide monomers is dissolved into the electrolyte when the oxidization-reduction is repeated. And, the dissolved monomers are polymerized to deposit at a site other than their original site in the electrode. The deposited polydisulfide compound, which is separated from an electrically conductive agent such as carbon black and polymerized, is apart from the electron/ion conductive network in the electrode and has no relation to the electrode reaction. If the oxidation-reduction is repeated, the number of the parted polydisulfide compounds is increased and the capacity of the battery is gradually decreased. The highly soluble organic disulfide monomer is easy to move and is dispersed from the cathode to a separator or into an electrolyte and, further, to the anode side. Therefore, the battery with the electrode comprising the organic disulfide compound as the cathode has disadvantages that the charging and discharging efficiency is deteriorated and cycle life of the charging and discharging is shortened.
Then the object of the present invention is to solve the above-mentioned problems and to provide a composite electrode, which does not lose the characteristics such as large capacity and high energy density of sulfur and proceeds the oxidation-reduction reaction rapidly even at room temperature.
Further, the object of the present invention is to provide a composite electrode which does not lose a high energy density of an organic sulfide compound and gives a high charging and discharging efficiency and good charging and discharging cycle life property.